When President Obama arrives in New York City today for the U.N. General Assembly, the world will be watching to see how he handles slow-boiling crises in Iran and Syria. Will he shake hands with Iran’s new President, Hassan Rouhani? Can he advance diplomacy aimed at bringing Syria’s civil war to an end?

The answers are not clear. But the fact that Iran and Syria are atop Obama’s agenda points to an emerging core truth about his presidency. Obama’s foreign policy has been a shifting and hard-to-define beast. But it’s clear that one priority drives him above all others: weapons of mass destruction.

The search for an Obama foreign policy doctrine has generally been a futile one. Obama took office in 2009 as a conciliator, extending a hand to rivals and offering a new era of cooperation. Then came his hardheaded pursuit of terrorists, expressed by his Afghanistan surge and his ruthless use of drone strikes. For a moment, his intervention in Libya had Washington buzzing about a new dawn of liberal interventionism. But then Obama spent months assiduously avoiding any involvement in Syria’s civil war, even as tens of thousands died.

No one thread has tied it all together. But the current diplomatic moment in New York City suggests—much like George W. Bush before him—Obama sees the proliferation of WMDs as the great overarching threat to America, and the most compelling basis for the use of military force.

The idea is laid out in the Obama Administration’s last official national-security strategy: “There is no greater threat to the American people than weapons of mass destruction,” the document explained.

That philosophy explains why Obama, after months of refusing to intervene in Syria, was on the brink of a military strike after Bashar Assad mounted a large-scale chemical-weapons attack this summer. Obama insisted that the U.S. simply couldn’t turn a blind eye to the use of WMDs anywhere. “A failure to stand against the use of chemical weapons would weaken prohibitions against other weapons of mass destruction,” Obama explained in his Sept. 10 address to the nation.

Obama has been even more emphatic when it comes to nuclear weapons. “The single biggest threat to U.S. security, both short term, medium term and long term, would be the possibility of a terrorist organization obtaining a nuclear weapon,” he said in April 2010.

That helps to explain Obama’s intense concern about Iran’s nuclear program, and his threat to use military force to stop it. Obama has warned not only about the dangers of a nuclear weapon in the hands of Iran’s anti-American (and anti-Israel) regime, but also of the risk that an Iranian bomb will touch off a nuclear arms race in the Middle East that could lead to dangerous proliferation of atomic weapons. “A nuclear arms race in the region is something that would be profoundly destabilizing,” Obama told ABC earlier this month. The U.S. has many strong disagreements with Iran—from its support for Hizballah to its refusal to recognize Israel—but it is the nuclear issue that has led Obama to impose tight sanctions and to rattle the military saber.

Concerns about nuclear weapons have driven Obama’s policies elsewhere. Secret documents recently leaked by Edward Snowden revealed the obsession America’s intelligence agencies have with Pakistan. They spend billions of dollars per year trying to learn details about Pakistan’s WMD arsenal, thought to include dozens of nuclear warheads but also chemical and biological weapons. Given the proximity of al-Qaeda-affiliated terrorists to these weapons, the Washington Post reported on Sept. 2: “No other nation draws as much scrutiny” as Pakistan. Obama’s “reset” policy with Russia—which has faltered of late—was initially driven by a push to reduce American and Russian nuclear stockpiles, a key step toward the goal of a nuclear-free world that Obama announced in his first speech abroad as President, in Prague, and which remains an important priority to him.

Meanwhile Obama has shown less interest in troubled countries that don’t pose a WMD threat. He happily withdrew U.S. troops from Iraq and is drawing down from Afghanistan, two nations with no nuclear, chemical or biological weapons.

In 2002, during the run-up to the Iraq war, the threat of weapons of mass destruction was a core theme of Bush’s speech to the U.N. General Assembly: “Our greatest fear is that terrorists will find a shortcut to their mad ambitions when an outlaw regime supplies them with the technologies to kill on a massive scale,” Bush said that September. More than a decade later, a very different President finds himself driven by much the same thinking as he heads to the U.N. this week.